Heretofore, for portable information terminal devices such as cell phones, flash memories are widely used as memory devices capable of memorizing in a nonvolatile manner. The reason is that high density integration of flash memory is relatively easy, production technique for flash memory is highly compatible with those of conventional logic or DRAM devices, and cost of flash memory is relatively low. However, it is not easy to form a thin film of insulator surrounding a floating gate of a flash memory.
In order to secure charge retention of floating gate, that is, reliability of memory retention of flash memory, it is necessary to make the film thickness of the insulator surrounding a floating gate at least a predetermined thickness. Thus, since forming of thin film of insulator is difficult, in a flash memory of conventional structure, progress toward high performance of device such as miniaturization, high speed, low voltage operation and low power consumption, becomes difficult, and there is a prediction that progress toward high performance reaches a limit as early as year 2007.
As a technique solving the above problem of securing reliability of charge retention and achieving a thin film of insulator at the same time in a single floating gate type flash memory, a technique of dividing the floating gate into a plurality of pieces, has been proposed. As an example of specific method for dividing a floating gate into a plurality of pieces, there is a method of forming a large number of Si ultrafine particles for each memory element and using a group of such Si ultrafine particles as a floating gate (for example, Patent Document 1). By thus dividing a floating gate, even when leakage of retained charge occurs, loss of accumulated electric charge can be limited to a small region, whereby requirement for reliability of insulator can be eased. A flash memory having such a floating gate divided into a plurality of pieces is referred to as a divided floating gate type flash memory, and the above-mentioned flash memory of conventional structure is referred to as a is single floating gate type flash memory in this document.
The technique of dividing a floating gate contributes to improvement of reliability of charge retention of an insulator, and enables forming of a thin film of insulator, and for this reason, the technique is one of techniques for solving the above-mentioned problem of conventional single floating gate type flash memory.
Further, a technique is studied, in which the ultrafine particles serving as a floating gate is not made of Si but made of a metal. It is known that a floating gate constituted by metal ultrafine particles can improve charge retention performance more than Si ultrafine particle floating gate (for example, Patent document 2). The reason is because metal has a work function relatively larger than the electron affirmity of Si, and accordingly, a potential barrier for electric charge retained in the metal ultrafine particle floating gate is higher than that for electric charge in the Si ultrafine particle floating gate.
Here, in the technique for forming a metal ultrafine particle floating gate disclosed in Patent Document 2, an insulation film (hereinafter referred to as gate insulation film) deposited between the metal ultrafine particle floating gate and a control gate, is formed by a CVD method, there is a case where the metal ultrafine particles are oxidized at a time of forming the gate insulation film. It is pointed out that when the metal ultrafine particles are thus oxidized, the number of metal ultrafine particles effectively working for charge retention may decrease.
As a technique for preventing such an oxidation of metal ultrafine particles, a technique of forming a gate insulation film by using a sputtering method is disclosed (for example, Patent Document 3). Since a sputtering method enables to form a film at a lower temperature as compared with a CVD method or a thermal oxidation method in general, and accordingly, it becomes possible to deposit a gate insulation film on the metal ultrafine particles while oxidation of the metal ultrafine particles is suppressed.
Patent Document 1: JP-A-11-186421
Patent Document 2: JP-A-2003-51498
Patent Document 3: JP-A-2003-86715